Significant fluid loss while drilling through fractured formations is a major problem for drilling operations. From field experience, we know that the type and rheological parameters of the drilling fluid have a strong impact upon the rate and volume of losses.

A mathematical model for Herschel-Bulkley [yield-power-law (YPL)] drilling-fluid losses in naturally fractured formations is presented. As a result, the effect of rheological properties of drilling fluid such as yield stress and shear-thinning/-thickening effect (flow-behavior index) on mud losses in fractured formations is investigated. We found that the yield stress can control the ultimate volume of losses while the shear-thinning effect can tremendously decrease the rate of losses. Therefore, mud losses in fractures can be minimized by optimizing the rheology of the drilling fluid properly.

The model also allows for quantitative analysis of losses that take into account fluid rheology to characterize the fractures. Hydraulic aperture of conductive fractures can be obtained by continuously monitoring mud losses and fitting field records of mud losses to the model. The proposed model is very useful not only for drilling applications but also for well-completion design and fractured-reservoir-characterization purposes. To examine the validity of the model, a practical application of the proposed technique is demonstrated through a field example of mud-loss measurements in a fractured well in the Gulf of Mexico.

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